Hydrogels are materials which absorb solvents (such as water), undergo rapid swelling without discernible dissolution, and maintain three-dimensional networks capable of reversible deformation (Park, et al., Biodegradable Hydrogels for Drug Delivery, Technomic Publishing Co., Lancaster, Pa., 1993; W. Shalaby et al., J. Controlled Rel., 19, 131, 1992; and Silberberg, in Molecular Basis of Polymer Networks (Baumgartner, A. & Picot, C. E., Eds.), Spring-Verlag, Berlin, 1989, p. 147).
Covalently crosslinked networks of hydrophilic polymers, including water-soluble polymers are traditionally denoted as hydrogels (or aquagels) in their hydrated state. Hydrogels have been prepared to be based on crosslinked polymeric chains of methoxy poly(ethylene glycol) monomethacrylate having variable lengths of the polyoxyethylene side chains, and their interaction as hydrogels, with blood components have been studied (Nagaoka, et al., in Polymers as Biomaterials (Shalaby, S. W., et al., Eds.), Plenum Press, 1983, p. 381). A number of aqueous hydrogels (aquagels) have been used in various biomedical applications, such as, for example, soft contact lenses, wound management, and drug delivery. However, methods used in the preparation of these hydrogels, and their conversion to useful articles, are subject to the constraints associated with the nature of their three-dimensional thermosetting structures and, hence, deprive the users from applying the facile processing techniques employed in the production of non-crosslinked thermoplastic materials.
This, and the low mechanical strength of the hydrated networks, led a number of investigators to explore the concept of combining hydrophilic and hydrophobic polymeric components in block (Okano, et al., J. Biomed. Mat. Research, 15, 393, 1981), or graft copolymeric structures (Onishi, et al., in Contemporary Topics in Polymer Science, (W. J. Bailey & T. Tsuruta, eds.), Plenum Publ. Co., New York, 1984, p. 149), and blends (Shah, Polymer, 28, 1212,1987; and U.S. Pat. No. 4,369,229) to form the "hydrophobic-hydrophilic" domain systems, which are suited for thermoplastic processing (Shah, Chap. 30, in Water Soluble Polymers (S. W. Shalaby, et al., Eds.), Vol. 467, ACS-Symp. Ser., Amer. Chem. Soc., Washington, 1991). The "hydrophobic-hydrophilic" domain system (HHDS) undergoes morphological changes which are associated with the hydration of the hydrophilic domains and formation of pseudo-crosslinks via the hydrophobic component of the system (Shah, 1991, cited above). Such morphology was considered to be responsible for the enhanced biocompatibility and superior mechanical strength of the two-phase HHDS as compared to those of covalently crosslinked, hydrophilic polymers. The mechanism of gel formation in the present invention parallels that described by Shah, 1991, cited above, for non-absorbable blends of hydrophilic-hydrophobic domain systems (HHDS). However, differences exist between the copolymers of the present invention, and more particularly, Component "A", and HHDS. In this regard, Component A is based on a water-soluble and water-insoluble block structure (SIBS). This is not a mere physical mixture of two polymers as are the blends described by Shah, 1991, cited above. Additionally, due to the presence of covalent links between the blocks of SIBS, the resulting hydrogel displays higher elasticity compliance and tensile strength while being absorbable. In fact, the SIBS systems are, in some respects, analogous to thermoreversible gels (Shalaby, in Water-Soluble Polymers, (Shalaby, S. W., et al., Eds.), Vol. 467, Chapt. 33, ACS Symp. Ser., Amer. Chem. Soc., Washington, D.C., 1991a) in displaying a hydration-dehydration equilibrium governing the system transformation, i.e., the gel/liquid equilibrium is driven by the water content of the SIBS. Thus, in the absence of water, the polyoxyalkylene blocks undergo intermolecular segmental mixing with the neighboring hydrophobic blocks to produce a viscous liquid. In the presence of water, competition between the water as an extrinsic solvent and the polyester block for the polyoxyalkylene (POA) block forces the hydration of the POA, and aggregation or association of the polyester blocks to establish pseudo-crosslinks which maintain a 3-dimensional integrity. Since gel formation takes place in an aqueous environment, the POA block will preferentially migrate to the exterior of the gel and interface with the adjoining tissues to establish an adhesive joint, which prevents gel migration from target site and sustains its intended efficacy. As for example, for periodontal and dry socket applications, post-surgical adhesion prevention and treatment of vaginal and bone infections, and other applications where predictable site residence of the gel cannot be compromised.
Synthesis and biomedical and pharmaceutical applications of absorbable or biodegradable hydrogels based on covalently crosslinked networks comprising polypeptide or polyester components as the enzymatically or hydrolytically labile components, respectively, have been described by a number of researchers (Jarreit, et. al., Trans. Soc. Biomater., Vol. XVIII, 182, 1995; Pathak, et. al., Macromolecules, 26, 581, 1993; Park, et. at., Biodegradable Hydrogels for Drug Delivery, Technomic Publishing Co., Lancaster, Pa., 1993; Park, Biomaterials, 9, 435, 1988; and W. Shalaby, et. al., 1992, cited elsewhere herein). The hydrogels most often cited in the literature are those made of water-soluble polymers, such as polyvinyl pyrrolidone, which have been crosslinked with naturally derived biodegradable components such as those based on albumin (Park, et. al., 1993, cited elsewhere herein; and W. Shalaby, et. al., 1992, cited elsewhere herein). Totally synthetic hydrogels which have been studied for controlled drug release and membranes for the treatment of post-surgical adhesion are based on covalent networks formed by the addition polymerization of acrylic-terminated, water-soluble chains of polyether dl-polylactide block copolymers (Jarrett, et. al., 1995, cited elsewhere herein; and Pathak, et al., 1993, cited elsewhere herein).
Polymer solutions which undergo reversible gelation by heating or cooling about certain temperatures (lower critical solution temperature, LCST) are known as thermoreversible gels. Theoretical and practical aspects of key forms of thermoreversible gels are described by Shalaby, 1991a, cited elsewhere herein. Among the thermoreversible gels discussed by Shalaby are those of amorphous N-substituted acrylamides in water and amorphous polystyrene and crystalline poly(4-methyl pentene) in organic solvents. Prevailing gel formation mechanisms include molecular clustering of amorphous polymers and selective crystallization of mixed phases of crystalline materials. Thermodynamic parameters (enthalpy and entropy) which favor gel formation in terms of LCST are discussed by Shalaby only with respect to the solvent-polymer interaction. Shalaby fails, however, to address self-solvating chains.
U.S. Pat. No. 4,911,926, discloses aqueous and non-aqueous compositions comprised of block polyoxyalkylene copolymers that form gels in the biologic environment, for preventing post-surgical adhesion. Other gel forming compositions for use in preventing post-surgical adhesion include: (a) chitin derivatives (U.S. Pat. No. 5,093,319); (b) aqueous solutions of xanthan gum (U.S. Pat. No. 4,994,277); (c) chitosan-coagulum (U.S. Pat. No. 4,532,134); and (d) hyaluronic acid (U.S. Pat. No. 4,141,973).
Absorbable polymers, or often referred to as biodegradable polymers, have been used clinically in sutures and allied surgical augmentation devices to eliminate the need for a second surgical procedure to remove functionally equivalent non-absorbable devices (U.S. Pat. No. 3,991,766, to Schmitt et al.; and Shalaby, in Encyclopedia of Pharmaceutical Technology (J. C. Boylan & J. Swarbrick, eds.), Vol. 1, Dekker, New York, 1988, p. 465). Although these devices were designed for repairing soft tissues, interest in using such transient systems, with or without biologically active components, in dental and orthopedic applications has grown significantly over the past few years. Such applications are disclosed in Bhatia, et. al., J. Biomater. Sci., Polym. Ed., 6(5), 435, 1994; U.S. Pat. No. 5,198,220, to Damani; U.S. Pat. No. 5,171,148, to Wasserman, et. al.; and U.S. Pat. No. 3,991,766, to Schmitt et al.
U.S. Pat. No. 3,991,766, to Schmitt et al., discloses absorbable articles made of polyglycolide acid, such as sutures, clips and storage pallets having medicaments incorporated therein and can be used for both their own mechanical properties and delayed release systems of medicaments. U.S. Pat. No. 5,171,148, to Wasserman et al., discloses the use of absorbable polymers made from p-dioxanone or L-lactide and glycolide as dental inserts for the treatment of periodontal disease. Here, a semiporous mesh material with sealed edges is emplaced between the tooth and gingiva. The implant is attached to the tooth by an absorbable ligature material. U.S. Pat. No. 5,198,220, to Damani, discloses the treatment of periodontal disease through the use of a sustained release composition/device comprising bioactive agents. The composition/device is in a liquid, semi-solid or solid form suitable for insertion into or around the periodontal pocket. Damani also teaches the formation of a gel, or paste, composition consisting of poly(lactyl-co-glycolide) in an acceptable solvent (such as propylene carbonate), with or without propylene and/or polyethylene glycol, and an antibiotic agent such as tetracycline hydrochloride.
Other in-situ forming biodegradable implants and methods of forming them are described in U.S. Pat. Nos. 5,278,201 ('201 Patent) and U.S. Pat. No. 5,077,049 ('049 Patent), to Dunn et al. The Dunn et al., patents disclose methods for assisting the restoration of periodontal tissue in a periodontal pocket and for retarding migration of epithelial cells along the root surface of a booth. The '049 Patent discloses methods which involve placement of an in-situ forming biodegradable barrier adjacent to the surface of the tooth. The barrier is microporous and includes pores of defined size and can include biologically active agents. The barrier formation is achieved by placing a liquid solution of a biodegradable polymer, such as poly(dl-lactide-co-glycolide) water-coagulatable, thermoplastic in a water miscible, non-toxic organic solvent such as N-methyl pyrrolidone (i.e., to achieve a typical polymer concentration of .ltoreq.50%) into the periodontal pocket. The organic solvent dissipates into the periodontal fluids and the biodegradable, water coagulatable polymer forms an in-situ solid biodegradable implant. The dissipation of solvent creates pores within the solid biodegradable implant to promote cell ingrowth. The '859 Patent likewise discloses methods for the same indications involving the formation of the biodegradable barrier from a liquid mixture of a biodegradable, curable thermosetting prepolymer, curing agent and water-soluble material such as salt, sugar, and water-soluble polymer. The curable thermosetting prepolymer is described as an acrylic-ester terminated absorbable polymer.
The '049 and '859 Patents, as well as U.S. Pat. No. 4,938,763 to Dunn et al., disclose polymer compositions primarily consisting of absorbable thermoplastic or thermosetting polymer, dissolved in organic solvent. These compositions are also described to produce, in an aqueous environment, solids which can be used as tissue barrier (Fujita, et. al., Trans. Soc. Biomater., Vol. XVII, 384, 1994) substrate for tissue generation (Dunn, et. al., Poly. Prepr., 35(2), 437, 1994a) or carrier for the controlled delivery of drugs (Sherman, et. al., Pharm. Res., 11(105-318, 1994). Acrylate-endcapped poly(caprolactone) prepolymer was also used as a branched precursor for the in-situ formation of a crosslinked system for potential use in controlled drug release (Moore, et. at., Trans. Soc. Biomater., Vol. XVIII, 186, 1995).
A number of controlled delivery systems for the treatment of periodontal disease are also described in the literature. For example, U.S. Pat. No. 4,919,939, to Baker, discloses a controlled release delivery system for placement in the periodontal pocket, gingival sulcus, tooth socket, wound or other cavity within the mouth. The system incorporates microparticles in fluid medium and is effective in the environment of use for up to 30 days. The drug, in 10-50 micron polymer particles, is released at a controlled rate by a combination of diffusion of the drug through the polymer and erosion of the polymer.
U.S. Pat. No. 5,135,752, to Snipes, discloses a buccal dosage form, which melts in the oral cavity, yet will not spontaneously deform at higher temperatures encountered in shipment and storage. This composition comprises two grades of polyethylene glycol, polyethylene oxide, long-chain saturated fatty acid, and colloidal silica.
U.S. Pat. No. 5,366,733, to Brizzolars et al., discloses an oral composition for the local administration of a therapeutic agent to a periodontal pocket comprising at least one therapeutic agent dispersed in a matrix including a biocompatible and/or biodegradable polymer. The composition is administered as a plurality of dry discrete microparticles, said microparticles are prepared by a phase separation process. An oral composition is also described wherein the polymer comprises a block copolymer of polyglycolide, trimethylene carbonate and polyethylene oxide. Apparatus and methods are also provided for dispensing the dry microparticles to the periodontal pocket, whereby they become tacky and adhere to the involved tissue so as to induce long-term therapeutic effects.
In addition, a number of systems for the controlled delivery of biologically active compounds to a variety of sites are disclosed in the literature. For Example, U.S. Patent No. 5,011,692, to Fujioka et al., discloses a sustained pulsewise release pharmaceutical preparation which comprises drug-containing polymeric material layers. The polymeric material layers contain the drug only in a slight amount, or free of the drug. The entire surface extends in a direction perpendicular to the layer plane and is coated with a polymeric material which is insoluble in water. These types of pulsewise-release pharmaceutical dosages are suitable for embedding beneath the skin.
U.S. Pat. No. 5,366,756, to Chesterfield et al., describes a method for preparing porous bioabsorbable surgical implant materials. The method comprises providing a quantity of particles of bioabsorbable implant material, and coating particles of bioabsorbable implant material with at least one growth factor. The implant can also contain antimicrobial agents.
U.S. Pat. No. 5,385,738, to Yamahira et al., discloses a sustained-release injection system, comprising a suspension of a powder comprised of an active ingredient and a pharmaceutically acceptable biodegradable carrier (e.g., proteins, polysaccharides, and synthetic high molecular weight compounds, preferably collagen, atelo collagen, gelatin, and a mixture thereof) in a viscous solvent (e.g., vegetable oils, polyethylene glycol, propylene glycol, silicone oil, and medium-chain fatty acid triglycerides) for injection. The active ingredient in the pharmaceutical formulation is incorporated into the biodegradable carrier in the following state: (i) the active ingredient is chemically bound to the carrier matrix; (ii) the active ingredient is bound to the carrier matrix by intermolecular action; or (iii) the active ingredient is physically embraced within the carrier matrix.
Furthermore, a common complication which is encountered by many surgeons following tooth extraction is dry socket. Dry socket occurs following three to four percent of routine extractions (Field, et. al., J. Oral Maxillofac. Surg., 23(6), 419, 1985), and its etiology appears to be multifactorial (Westerholm, Gen. Dent., July-Aug., 306, 1988). Over the years, dry socket has been referred to as alveoloalgia, alveolitis sicca dolorosa, avascular socket, localized osteitis, fibrinolytic alveolitis and localized acute alveolar osteomyelitis (Shafer, et al., A Textbook of Oral Pathology, 4th Ed., W. B. Saunders Co., Philadelphia, 1974, p. 605, 1974; and Birn, Int. J. Oral Surg., 2, 211, 1973). Although many chemotherapeutic prevention measures or management have been pursued, none have significantly reduced the incidence of dry socket (Birn, 1973, cited above; Field, el. al., 1985, cited above). Among such approaches to the therapeutic treatment of dry socket, with limited success, are those based on systemic administration of antibiotics (Westerholm, 1988, cited above) or direct placement of powdered sulfadiazine or sulfathiazole into the socket (Elwell, J. Amer. Dent. Assoc., 31, 615, 1944).
To date, the known HHDS and thermoreversible gels can be classified as non-absorbable materials and are expected not to absorb through chain dissociation in the biological environment. Meanwhile, there is a growing interest in developing absorbable sutures and allied surgical devices such as transient implants, which are degraded to bioabsorbable, safe by-products and leave no residual mass at the surgical site, as well as frequently cited clinical advantages (Shalaby, Chap. 3 in High Technology Fibers (M. Lewin & J. Preston, eds.), Dekker, New York, 1985; Shalaby, 1988, cited elsewhere herein; Shalaby, Polym. News, 16, 238, 1991; Shalaby, J. Appl. Biomater., 3, 73, 1992; Shalaby, Biomedical Polymers: Designed to Degrade Systems, Hanser Publ., New York, 1994; and Shalaby, et al, eds. Polymers of Biological & Biomedical Significance, Vol. 520, ACS-Symp. Ser., Amer. Chem. Soc., Washington, 1993) have justified the need for novel absorbable hydrogel formulations.
Moreover, such systems as those previously described in the literature, for example, such as by Dunn, et al, (U.S. Pat. No. 4,938,763), teach in-situ formations of biodegradable, microporous, solid implants in a living body through coagulation of a solution of a polymer in an organic solvent such as N-methyl-2-pyrrolidine. However, the use of solvents, including those of low molecular organic ones, facilitates migration of the solution from the application site thereby causing damage to living tissue including cell dehydration and necrosis. Loss of the solvent mass can lead to shrinkage of the coagulum and separation from surrounding tissue.
Furthermore, currently available drug delivery systems deal with solid implants which can elicit mechanical incompatibility and, hence, patient discomfort. The present invention provides novel, hydrogel-forming copolymers, which in contrast to those systems previously described, are absorbable, do not require the use of solvents, and are compliant, swollen, mechanically compatible gels, which adhere to surrounding tissue.